A σ-Donor with a Planar Six-π-Electron B 2 N 2 C 2 Framework: Anionic N-heterocyclic Carbene or Heterocyclic Terphenyl Anion? 

The remarkable success of N-heterocyclic carbenes [1] (NHCs) as ligands in main group and especially in transition metal chemistry and catalysis [2] has prompted the synthesis and characterization of numerous representatives of this class of compounds. Derivatives


Dedicated to Professor Tristram Chivers
The remarkable success of N-heterocyclic carbenes [1] (NHCs) as ligands in main group and especially in transition metal chemistry and catalysis [2] has prompted the synthesis and characterization of numerous representatives of this class of compounds.Derivatives with a five-membered framework (A) have been investigated most extensively.However four-, six-and seven-membered carbenes were also reported.Emerging trends in carbene ligand design involve the replacement of one or both intraannular nitrogen atoms with elements such as carbon, phosphorus, and sulphur, [3] and the replacement of the carbon backbone with heteroelements (B -D). [4]n this way, the steric and electronic properties of the ligand can be effectively tuned.Although NHCs are traditionally neutral ligands, a few anionic derivatives have been prepared in which the charge is localized on an adjoining borate or cyclopentadienyl moiety. [5]erein we report the synthesis of an anionic NHC (3) having a six π-electron system with the charge localized on the ring framework.This compound has the potential to coordinate in either σ (η 1 ) or π (η 6 ) fashion.The reaction of the corresponding trimethylsilyl formamidinate [4c] with 1,1-bis(methylchloroboryl)ethane [6] in the presence of trimethylsilyl triflate (Scheme 1) yielded the zwitterionic ring compound 1.Crystallographic analysis (Figure 1) revealed an iminium borate structure with the triflate coordinated to one of the boron atoms. [7]The B2N2C2 ring in 1 has a half-boat conformation with the sp 3 hybridized carbon C(2) situated outside of the slightly Scheme 1. Synthesis of derivatives 1 -3.Deprotonation of 1 with an equimolar amount of K[N(SiMe3)2] cleanly yielded the 1,5-diaza-2,4-diborine 2. Few carbaborazines have been described, and a derivative with this framework has not been reported; however its stability and properties ave been the object of theoretical calculations. [8]At -40 °C, the 13 C NMR spectrum featured signals corresponding to the ring carbon atoms adjacent to nitrogen and boron at  = 146.8and 119.2 ppm, respectively, in the typical range for benzene derivatives.
The X-ray structural determination of 2 (Figure 2) confirmed the proposed structure, which contains a planar B2N2C2 ring with equivalent pairs of B-N, B-C and N-C bond lengths. [7]The endocyclic N-C bonds lengths (ca.1.33 Å) are intermediate in length between the two values observed in 1, and practically identical to the bonds observed in the cationic precursors to carbenes B -D. [4] The B-N bond lengths in 2 (1.50 Å) are similar to the those observed in C and significantly longer than the those in borazines (1.42 -1.44 Å). [9] The intraannular B-C and N-C bonds (1.48 and 1.33 Å, respectively) are shorter than the corresponding bonds observed in the regioisomers 1,3-diaza-2,4-diborine and 1,4-diaza-2,3-diborine (ca. 1.53 and 1.38-1.40 =, respectively). [10]The B-N bonds in the latter derivatives are equal with those in borazines.As proposed by Bertrand and co-workers for the cationic precursor to D, [4d] the six π electrons in 2 appear to be distributed onto two allyllike fragments (B2C -and N2C + ) to form a zwitterionic structure, rather than delocalized over the entire ring framework.Deprotonation of 2 with nBuLi produced the lithium salt 3, which was stable in solid state under an inert atmosphere but decomposed slowly in tetrahydrofuran (thf) through deprotonation of the solvent and reformation of the starting material.The crystallographic determination (Figure 3) revealed that 3 contains a planar B2N2C2 ligand σ-coordinated to the lithium ion. [7]The distorted trigonal planar coordination environment of lithium is completed by two thf molecules, and the ring carbon atoms and the lithium atom lie on a crystallographic C2 axis.Consistent with other NHCs, deprotonation of 2 results in lengthening of the intraannular N-C bonds by approximately 0.03 Å to a value of 1.36 Å.The N(1)-C(1)-N(1') angle in 3 (114.0°) is more acute than in 2 (122.8°) and 1 (125.7°).This is a result of a lateral compression of the ring skeleton, causing an increase of the C(1)•••C(2) distance of 0.13 Å in 3 versus 2. The overall geometry of 3 strongly resembles the structure of isoelectronic lithium terphenyl derivatives. [11]The planar six-πelectron σ-donating anion in 3 could therefore be seen as either an anionic NHC or a heterocyclic terphenyl anion (E).
The Li-C bond length and the chemical shift of the carbon atom connected to lithium can be considered when comparing 3 to a lithium NHC complex and a lithium terphenyl.The Li(1)-C(1) bond measures 2.152(6) Å, which is typical for lithium NHC complexes.
In representatives of this class, the substitution of the lithium center or the charge of the NHC ligand have little influence on the length of the Li-C bond (2.124(4) -2.197(4) Å). [12] Conversely, the length of the Li-C bond in trigonal-planar, three-coordinate terphenyl lithium derivatives falls within a narrow range (2.074(16) -2.128(4) Å). [13] The 13 C NMR signal corresponding to the carbene carbon could not be located in the spectrum of 3.However, the potassium analog, obtained by deprotonation of 2 with benzyl potassium, displayed a signal for the carbene carbon at 239.1 ppm, which is significantly downfield shifted in comparison to the ipso carbon in terphenyl lithium derivatives (175 -201 ppm) [13] and the carbene carbon atom in NHC-Li (189 -198 ppm) [12] and NHC-K complexes [14] (199 -208 ppm).It is uncertain if this signal belongs to the free anionic carbene or the potassium-coordinated ligand.To our knowledge, potassium terphenyl derivatives have not been reported.
A density functional theory treatment of the simplified structures I -III showed an increase in the energy of both the highest occupied molecular orbitals (HOMOs; σ) and the lowest unoccupied molecular orbitals (LUMOs; π) in the order of I to II to III, which indicates that the σ-donating ability decreases in order III > II >> I and the π-accepting ability decreases in reverse order (Figure 4).NHCs are generally considered to be excellent σ-donors and poor π-acceptors.Analysis of the electron localization function (ELF) [15] for the compounds in question gives occupancies of 2.28, 2.35, and 2.45 electrons for the carbon lone pair basins in III, II and I, respectively.Hence, the ELF analysis suggests that NHC I is a better σ-donor than is predicted by the orbital energies alone.The same is implied by the trends in Mulliken atomic charges, which show that the carbene carbon atom in NHC I bears slightly more negative charge (-0.20) than that in II (-0.15) or III (-0.14).To fully resolve the issue of relative -donating ability of the studied systems, we performed charge decomposition analyses (CDA) [16] for complexes of I-III with Li + and CuCl.The results show that the donating ability of ligands I-III follows the trend in orbital energies for the Li + complexes (total donation of  electrons is 0.484, 0.566, and 0.622 for I, II, and III, respectively), but within the CuCl series ligand II has similar -donating ability to I (total donation of 0.236 and 0.233 electrons, respectively) and III has slightly less (0.206 electrons).Hence, the computational data indicate that the new anionic carbene II has similar or even greater -donating ability, depending on the metal fragment, than the traditional ligand I.
In summary, experimental and theoretical evidence indicates that the anionic carbene in 3 bridges the gap between two classical systems that have quite different properties: NHCs and terphenyl anions.Its facile synthesis from the formally zwitterionic diazadiborine 2 renders the novel system a promising anionic ligand, with which both σ (η 1 ) and π (η 3 or η 6 ) coordination modes are possible.Its coordinating ability towards transition metals is currently being investigated.

Experimental Section
All operations were carried out with exclusion of air and moisture.The NMR spectra were run on a Bruker Avance DRX-400 spectrometer.All chemicals were prepared according to reported procedures or purchased from commercial sources.Computational details are presented in Supporting Information.

Figure 4 .
Figure 4. Frontier molecular orbitals and ELF diagrams for systems I -III.